1. Field of the Invention
The invention relates generally to semiconductor manufacturing and more specifically to a cleaning method and apparatus for a single-wafer cleaning system which minimizes galvanic corrosion.
2. Description of the Related Art
Galvanic corrosion is induced in an environment where two dissimilar metals are coupled through an electrolyte. One of the metals in the galvanic cell becomes an anode and corrodes faster than it would normally, while the other metal becomes a cathode and corrodes slower than it would normally. FIG. 1 illustrates prior art diagram 100 of a basic galvanic cell. Two dissimilar metals, 104 and 106, are coupled through electrolyte 102. The anode 106 donates electrons and has its corrosion rate increased while the cathode 104 has its corrosion rate reduced.
Metal interconnects used in semiconductors are often constructed from dissimilar metals such as Copper/Tantalum (Cu/Ta) or Copper/Tantalum Nitride (Cu/TaN). During cleaning operations following processing operations such as etch and chemical mechanical planarization (CMP), the dissimilar metals are brought into electrical contact through an electrolyte, such as water from an aqueous based cleaner or a semi-aqueous based cleaner. As a result, corrosion of one of the metals is accelerated, thereby creating the potential for device failure. FIG. 2 illustrates prior art diagram 110 depicting one example of where dissimilar metals can form a galvanic cell. Diagram 110 illustrates a dual damascene structure where trench 120 includes a via 122 down to copper metallization line 112. Liner 114 encases copper metalization line 112 around 3 sides and acts a copper diffusion barrier. Dielectric layer 118 is typically a low K dielectric disposed over barrier 116. As can be seen in diagram 110 the via 122 is slightly misaligned over copper metalization line 112. Consequently, two dissimilar metals are exposed, the copper of copper metallization line 112 and the liner 114 since liner 114 is typically tantalum or tantalum nitride for dual damascene applications. An additional misaligned via on a second metal line (not shown), which is not in contact with the metallization line 112, can also introduce the potential for a galvanic cell once the dissimilar metals exposed in isolated lines are brought into contact through an electrolyte. It should be appreciated that the via need not be misaligned as the copper can be brought into contact with a second metal exposed in a different region of substrate 124. Thus dissimilar metals of even perfectly aligned structures can be brought into contact through an electrolyte during cleaning and rinsing operations. Additionally, while a dual damascene structure is presented in diagram 110, traditional metallization processes using aluminum can also create the potential for a galvanic cell.
During cleaning operations, substrates are exposed to cleaning chemistries. In the case of single-wafer cleaning operations the cleaning chemistries are formulated to be fast acting and the stoichiometry of the components is critical to the performance of the cleaning chemistry. For example, semi-aqueous cleaning chemicals for single-wafer cleaning operations typically include a solvent to remove organic material, a chelator to enhance metal contaminant removal from surfaces exposed to sputtering from the etch process, and a surfactant to passivate sensitive surfaces, especially those vulnerable to corrosion. Examples of commercially available single-wafer cleaning chemistries used for post via etch applications include NE-89 from Ashland Inc. of Dublin, Ohio and EKC 640 from EKC Technology, Inc. of Hayward, Calif.
The surfactant of the cleaning chemicals for the single-wafer cleaning operations are formulated to help improve wetting of difficult-to-access features such as vias and contacts, and also to control galvanic effects where necessary, however, if the surfactant is diluted then its passivation capacity is reduced or inhibited, thereby leaving the substrate more vulnerable to galvanic corrosion effects. For example, where the cleaning chemistry is puddled on the substrate and then rinsed off with de-ionized (DI) water, the water acts as an electrolyte to initiate the mechanism for galvanic corrosion. The galvanic corrosion may occur within the first few seconds of rinsing, where the cleaning chemistry and the surfactant are initially diluted upon rinsing of the cleaning chemistry. The dilution of the cleaning chemistry upsets a chemical equilibrium established to protect the substrate surface from corrosion. Since the surfactant concentration is modified by dilution through rinsing, the semiconductor substrate is vulnerable to corrosion when the diluted surfactant concentration is insufficient to inhibit corrosion.
FIG. 3 illustrates a prior art diagram displaying the various concentration gradient regions formed during the rinsing operations from a vantage point above the substrate 126. Substrate 126 is spinning in the direction of arrow 134. Region 128 depicts the region containing the cleaning chemistry puddled onto the substrate 126 through a nozzle or other delivery mechanism (not shown). To rinse of the cleaning chemistry from the substrate 126, DI water is sprayed onto the substrate 126 through a nozzle (not shown) directed toward the outer edge of the substrate 126 while the substrate is spinning. As the DI water is sprayed on the substrate 126, regions of differing gradients will form on the substrate 126. Region 130 contains a mixture of the cleaning chemistry and DI water, which forms as the DI water is initially sprayed onto the substrate 126. After a period of time, enough DI water is sprayed onto the substrate 126 where the cleaning chemistry is displaced and region 132 containing only DI water forms. While FIG. 3 provides a snapshot of one instance during the rinsing process, it should be appreciated that the edges of regions 132 and 130 are moving toward the edge of substrate 126 as depicted by arrows 136. The DI water rinse continues until eventually all of the cleaning chemistry is displaced from the substrate 126.
As mentioned above, region 130 includes a mixture of cleaning chemistry and DI water. Thus, the chemical equilibrium under which the cleaning chemistry is designed to function has been shifted. As a result of the dilution of the surfactant by the DI water, the corrosion protection of the surfactant is inhibited, which in turn exposes the substrate 126 to the effects of galvanic corrosion. As mentioned above, the effects of corrosion, especially galvanic corrosion, can occur within seconds.
In view of the foregoing, there is a need to provide an apparatus and method to rinse the cleaning chemistry from a substrate in a manner which protects the exposed metals of the substrate from galvanic corrosion.